Jinja2 syntax highlighting + description field + preview toggle

JinjaEditor:
- Custom StreamLanguage parser for Jinja2 syntax highlighting:
  {{ variables }} in blue, {% statements %} in purple, {# comments #} in gray
- Replaced HTML mode (didn't understand Jinja2 syntax)
- Proper monospace font (Consolas/Monaco)

TemplateConfig:
- Added `description` field to model + seed defaults with descriptions
- Description shown on template cards instead of raw template text
- Description input in create/edit form

Preview:
- Toggle behavior: clicking Preview again hides the preview
- Per-slot preview uses preview-raw API (renders current editor content)

i18n: added common.description, templateConfig.descriptionPlaceholder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -30,6 +30,7 @@
 	},
 	"dependencies": {
 		"@codemirror/lang-html": "^6.4.11",
+		"@codemirror/language": "^6.12.2",
 		"@codemirror/state": "^6.6.0",
 		"@codemirror/theme-one-dark": "^6.1.3",
 		"@codemirror/view": "^6.40.0",